Polyethylene is one of the most common thermoplastic polymers, used in a wide range of applications, and it is produced using ethylene as a monomer. Ethylene is generally produced via cracking of crude oil derivatives; in fact, it is common to find petrochemical complexes where the refinery, cracker and polymer plant are located on a single site. This is also due to the fact that polyethylene production requires quite high capital investments, while the final consumer product is commonly sold at relatively low price. Hence, improving polymer production techniques in order to reduce the manufacturing costs continues to be an important area of research, development and process improvement.
Various technologies for the production of polyethylene are commercially available; among these, gas-phase polymerization processes are the latest technologies developed, and are commonly employed in the production of high density polyethylene (HDPE), medium density polyethylene (MDPE) and linear-low density polyethylene (LLDPE). In gas-phase polymerization processes, olefin polymerization is carried out in a gaseous medium in the presence of a solid catalyst based on a transition metal compound belonging to the groups IV, V or VI of the periodic table, and suitable cocatalysts.
An example of said gas-phase polymerization processes involves the use of a fluidized bed reactor, wherein a bed of polymer particles is maintained in a fluidized state by the upward flow of gaseous monomer. The reactor generally consists of a reaction zone, in which the polymer particles are maintained in a fluidized state by passing a gaseous reaction mixture containing olefin(s) and optionally an inert gas through a bed of polymer particles. The catalyst is introduced in the reactor and the polymer constituting the fluidised bed is also removed.
A gas distribution grid placed in the lower part of the reactor under the reaction zone is the means through which the fluidisation gas is sent through the polymer bed and is used to support the bed itself when the polymerization is discontinued.
The gaseous mixture, comprising monomers, comonomers, inert gas and molecular weight regulators, leaving the top of the reactor is sent to the reactor at a point below the gas distribution grid through a recycling line. Devices for the compression and cooling of the gases are generally arranged on said recycling line. Make-up monomers are usually fed in the gas recycling line in such a way to have a certain homogeneity of the gaseous mixture inside the reactor.
In industrial gas-phase processes, it has often been observed that plant operability increasingly reduces with time, rendering the process non-economical after a few years due to more frequent maintenance needs. The reduced operability is commonly accompanied by an increase in electrostatic charges in the reactor, and by the consequent formation of lumps. In fact, as a result of electrostatic forces, the catalyst and the polymer particles tend to adhere to the reactor walls. If the polymer remains in a reactive environment for a long time, excess temperature can result in particle fusion with the formation of sheets or layers of thin fused agglomerates in the granular product. There is a strong correlation between sheeting and the presence of excessive electrostatic charges (either negative or positive). This is evidenced by sudden changes in electrostatic levels followed closely by deviation in temperature at the reactor wall. The temperature deviations indicate particle adhesion, which causes an insulating effect and poorer heat transfer from the bed temperature. As a result, there is generally disruption in the fluidization patterns, catalyst feed interruption can occur, as well as plugging at the product discharge system.
There are numerous causes for the formation of electrostatic charges, including the friction of dissimilar materials, a limited static dissipation and an excessive catalyst activity.
The decrease in operability may also be due to the presence of poisons in the catalyst system (e.g., in the catalyst support, the catalyst itself or the co-catalyst), the presence of contaminants in solvents or additives used during the polymerization reaction, or the presence of contaminants in the reactant feeds, such as monomer or hydrogen feeds.
The presence of contaminants and poisons generally lead to a decrease in process efficiency and plant productivity, both for the need of stopping the production for cleaning the plant, and for the reduced polymerization rate. Moreover, such contaminants may affect the specifications of products.
Acetylene, carbon monoxide and carbon dioxide are by-products of the pyrolysis process to make ethylene; while the high pressure and slurry polymerization processes are much less affected by them, gas-phase polymerization processes, and in particular fluidized bed processes, are very sensitive to these poisons which, even in small amounts, are able to reduce significantly the polymerization rate.
To ensure that ethylene feeds are free of poisons, an ethylene purification train comprising a series of catalytic beds is commonly used to remove e.g. acetylene, carbon monoxide and oxygen, followed by molecular sieves to remove moisture and carbon dioxide. For the same reason, comonomers also require a purification step. Comonomers are usually degassed by means of a stripper column and dried on molecular sieves. The purification train poses a capital investment cost and operating costs that are incurred to a much lower extent in high pressure and slurry processes.
Therefore, it would be desirable to develop a gas-phase polymerization process able to guarantee a constant operability and production levels of the polymerization plants over long service times, without the need for time consuming and expensive plant shutdowns for maintenance services.